Conventionally, there is known an internal-magnet type speaker device including a magnetic circuit having a pot type yoke and a planar plate and a vibrating system including one or more damper, a voice coil bobbin and a voice coil.
Generally, in such the speaker device, the voice coil is wound around an area in the vicinity of a lower end of the voice coil bobbin. The area in the vicinity of the lower end of an inner peripheral wall of the voice coil bobbin is opposed to an outer peripheral wall of the plate fixed on the pot type yoke with a fixed space from each other. At the same time, the area in the vicinity of the lower end of the outer peripheral wall of the voice coil bobbin is opposed to an area in the vicinity of an upper end of a cylindrical portion of the pot type yoke with a fixed space from each other. One or more damper, which is arranged at a position above the voice coil, is fixed to the area in the vicinity of the upper end of the outer peripheral wall of the voice coil bobbin.
In such the speaker device, at the time of driving, the vibrating system such as the voice coil and the voice coil bobbin vibrates in an axial direction of the speaker device. At this time, a balance of the vibrating system is lost due to various kinds of factors, and the voice coil bobbin sometimes rolls. Rolling (to roll) means that a power affecting the voice coil bobbin becomes asymmetry due to some kind of cause and the voice coil bobbin and the like roll in the lateral direction with respect to an original vibrating direction, i.e., the axial direction of the speaker device.
Particularly, in the above-mentioned speaker device, since the damper is arranged at the position above the voice coil, a fulcrum of the rolling is located at the position above the voice coil. Therefore, a distance from the fulcrum to the area in the vicinity of the lower end of the voice coil bobbin around which the voice coil is wound becomes longer, and the lower end portion of the voice coil bobbin widely rolls (rolling) in the lateral direction with the fulcrum as its center. As a result, the voice coil contacts the pot type yoke, or the voice coil bobbin contacts the plate, and an abnormal sound problematically occurs.
There are proposed various kinds of speaker devices for decreasing the rolling phenomena. For example, there is proposed a speaker in which a mechanical filter is arranged on an upper portion of a voice coil bobbin and the mechanical filter and a diaphragm are bonded at one point on a central axi O-O (see Japanese Patent Application Laid-Open under No. 8-51692). There is also proposed a speaker for suppressing the rolling of a diaphragm and a voice coil by supporting the voice coil by a damper and a cylindrical-shape cushion (see Japanese Patent Application Laid-Open under No. 10-42392).